Continuous inkjet (CIJ) printing systems create printed materials by forcing ink, under pressure, through a nozzle. The flow of ink may be disrupted in a manner such that the flow breaks up into droplets of ink in a predictable manner. Printing occurs through the selective deflecting and catching of undesired ink droplets. In U.S. Pat. No. 6,273,559 filed in the names of Vago et al. there are described continuous inkjet printing techniques one of which is referred to as the binary continuous inkjet technique. In the binary continuous inkjet technique electrically conducting ink is pressurized and discharged through a calibrated nozzle and the ink jets formed are broken off at two different time intervals. Droplets to be printed or not printed are created with periodic stimulation pulses at a nozzle. The droplets to be printed are each created with a periodic stimulation pulse that is relatively strong and causes the ink jet stream forming that droplet to separate at a relatively short break off length. The droplets that are not to be printed are each created with a periodic stimulation pulse that is relatively weak and causes the droplet to separate at a relatively long break off length. Electrodes are positioned just downstream of the nozzle and provide a charge to each droplet that is formed. The longer break off length droplets are selectively deviated from their path by a deflection device because of their charge and are deflected by the deflection device towards a catcher surface where they are collected in a gutter and returned to a reservoir for reuse.
The binary CIJ printheads may be operable in a manner such that the liquid jets may be said to have associated therewith a wavelength λ that is the distance between successive ink droplets or ink nodes in that liquid jet. The wavelength, λ, is equal to the speed of the jet divided by the frequency of the stimulation signals, assuming one stimulation signal at each nozzle during a stimulation cycle. It is thus possible to modulate the liquid jets break off points such that there exist a first and a second liquid break off points such that the break off points differ by a distance measured related to this wavelength. For example, in the aforementioned Vago et al. patent the longer and shorter break off length droplets have a distance between two jet break off points of less than λ. The longer break off length droplets have a break off point or droplet formation point d2 that is spaced from the location d1 where the shorter break off length droplets form by a distance less than λ. In Vago et al. there is mention made of prior art wherein the delta difference between d2 and d1 is λ and that this creates problems when there is a transition at a nozzle from creation of a longer break off length droplet followed by a shorter break off length droplet. The problem recognized by Vago et al. is that of the tendency of the longer break off length droplet and the shorter break off length droplet to simultaneously detach; i.e. two droplets break off from the jet concurrently. Where the delta difference is slightly greater than λ the two droplets may temporarily be combined and alter the trajectory of the droplets. There is thus the strong suggestion by Vago et al. to avoid the use of having droplet separation distance differences between the longer break off length droplets and shorter break off length droplets be greater than or equal to λ. To this end the specification of Vago et al. is directed to the teaching of using a significantly smaller break off separation distance between the longer break off length droplets and the shorter break off length droplets.
To enable droplet selection based on such small break off length differences as taught by Vago et al. it is necessary to establish electric fields having a sharp gradient along the jet trajectory. Vago et al. is able to achieve these high gradients by utilizing two sets of charge plates that were closely spaced along the drop trajectory. One of the electrode pairs was biased at +300 volts relative to the drop generator and the second electrode pair biased to −300 volts relative to the drop generator. To alter the break off length locations as described in the Vago et al. specification requires two stimulation amplitudes, a print and a non-print stimulation amplitude, to be employed. Limiting the break off length locations difference to less than λ restricts the stimulation amplitudes difference that must be used to a small amount. This amplitude control is quite easy to employ to separate print and nonprint droplets for a printhead that has only a single jet. However, in a printhead having an array of nozzles it is common for there to be variations in stimulation response from nozzle to nozzle so that different nozzles require different stimulation amplitudes to produce a particular break off length location. In an array of many nozzles, the variations in stimulation from nozzle to nozzle can exceed the difference in amplitude from long to short droplet break off locations for a jet. In such systems extra control complexity is required to adjust the stimulation amplitude from nozzle to nozzle while allowing a change in amplitude from a base level to produce the desired change in break off length.
It is therefore an object of the invention to overcome the aforesaid deficiencies by allowing the change in break off length from long break off length to short break off length to be greater than λ. This enables the use of less complex charge electrode structures and larger spacing between the charge electrode structures and the nozzles.